1. Field of the Invention
This invention relates to a defect inspecting apparatus, and particularly to a defect inspecting apparatus suitable for use in detecting the defect of a circuit pattern or a reticle or a photomask used as a negative in the manufacture of a semiconductive element or the like, or defects such as foreign substances on a substrate such as a semiconductive wafer.
2. Related Background Art
A defect inspecting apparatus is used in detecting, for example, the defect of a circuit pattern on a reticle or a photomask used as a negative in the manufacture of a semiconductive element or the like, or foreign substances on a substrate such as a semiconductive wafer.
FIG. 35A of the accompanying drawings shows a defect inspecting apparatus according to the prior art, and this defect inspecting apparatus is used to detect, for example, defects (including foreign substances) present on a semiconductive wafer. The wafer is provided with a number of redundant circuit patterns having periodical structure. FIG. 35B of the accompanying drawings shows a semiconductive wafer 1 which is the object of inspection. The semiconductive wafer 1 generally includes a regular array of circuit units (hereinafter referred to as the "dies") 2. Each die 2 has at least several tens of redundant circuit patterns along x axis and y axis, respectively. Each die 2 typically is a square having a side of about 20 mm.
In FIG. 35A, the inspecting apparatus is provided with a laser source 3, and a monochromatic laser beam emitted from the laser source 3 is converted into a substantially parallel beam of light 5 having a predetermined diameter by a beam expander 4. The beam of light 5 is then converged on a focus 7 positioned in the rearward focal plane of a lens 6, by the lens 6. A beam of light 8 diverging from the focus 7 is reflected by a small reflecting mirror 9 located near the focus 7. A beam of light of a circular cross-sectional shape reflected by the reflecting mirror 9 travels toward a Fourier transform lens 10.
The lens 10 is located such that the distance from the reflecting mirror 9 to the actually effective center of the lens 10 is a distance slightly smaller than one time the focal length of the lens 10. A parallel beam of light 11 emerging from the lens 10 is projected onto the surface of the semiconductive wafer 1 on which the patterns are formed. The semiconductive wafer 1 is held in a chuck 13 forming a part of two-dimensional parallel-moving means 12. The two-dimensional parallel-moving means 12 can move the semiconductive wafer 1 two-dimensionally in a plane perpendicular to the optical axis of the lens 10. The semiconductive wafer 1 is located in the object plane (i.e., the forward focal plane) 14 of the lens 10, and the parallel beam of light 11 illuminates the surface of the semiconductive wafer 1 on which the patterns are formed. In the optical system of FIG. 35A, the reflecting mirror 9 is a half mirror.
The portion 206 of FIG. 35A shows the manner in which the object plane 14 of the lens 10 is seen from above (from Z direction). As shown in the portion 206 of FIG. 35A, the parallel beam of light 11 illuminates the illuminated area 15 of a diameter 20 mm on the surface of the semiconductive wafer 1. As shown in FIG. 35A, a beam of light 16 diffracted by the illuminated area on the semiconductive wafer 1 is directed to the Fourier transform plane (i.e., the rearward focal plane) 17 of the lens 10 by the lens 10. The Fourier transform patterns of the circuit patterns in the illuminated area on the surface of the semiconductive wafer 1 are imaged in the Fourier transform plane 17.
Where the semiconductive wafer 1 is a wafer having no circuit (i.e., a wafer on which the circuit patterns are not yet formed), it is as shown in FIG. 36 of the accompanying drawings. FIG. 36 is a schematic view of the essential portions of the inspecting apparatus of FIG. 35A. In the inspecting apparatus of FIG. 36, the diameter of the beam spot at the rearward focus of the lens 6 is in inverse proportional relation with the diameter of the monochromatic parallel beam of light 5 projected from the beam expander 4 onto the lens 6. The beam of light 8 diverging from the focus 7 travels toward a half mirror 18, and the beam of light having a circular cross-section reflected by the half mirror 18 travels toward the Fourier transform lens 10. The parallel beam of light 11 emerging from the lens 10 is reflected by the wafer 1A having no circuit, whereafter it is again transmitted through the lens 10 and becomes a beam of light 19, which forms a beam spot 20 on the Fourier transform plane 17 of the lens 10. The beam diameter of the beam spot 20 becomes substantially equal to the beam diameter at the focus 7.
Turning back to FIGS. 35A and 35B, the illuminated area 15 of a diameter 20 mm on the semiconductive wafer 1 provides a Fourier transform pattern of sufficient accuracy. The reason is that the semiconductive wafer 1 comprises a number of redundant circuit patterns.
Also, a space filter 21 prepared in advance is disposed in the Fourier transform plane (the rearward focal plane) 17 of the Fourier transform lens 10. The space filter 21 can be made by exposing a recording medium like a photographic dry plate to light diffracted by all dies 2 of the semiconductive wafer 1. This can be done by the use of the semiconductive wafer 1 to be inspected. The reason is that even if the patterns on the semiconductive wafer 1 include defects, defect information will be conveyed by light of relatively low intensity and the light of low intensity including the defect information will not expose the photographic dry plate, whereas Fourier transform information of relatively high intensity from the original pattern on the semiconductive wafer 1 (light from the pattern) will expose the photographic dry plate.
Accordingly, the space filter 21 blocks the spatial frequency of the errorless Fourier transform information of the illuminated dies 2 on the semiconductive wafer (the Fourier transform information from the patterns free of defect), but passes therethrough the light created from the defect in these dies 2. The beam of light 22 conveying the defect information which is not blocked by the space filter 21 enters a reverse Fourier transform lens 23. This lens 23 is shown as a single lens, but includes a case where it is comprised of a plurality of lens elements. Also the lens 23 is located at a distance one time as great as the focal length of the lens 23 from the Fourier transform plane 17 of the lens 10. The lenses 10 and 23 are aligned along the same optical path 24, and the two-dimensional parallel-moving means 12 moves the semiconductive wafer 1 in a direction intersecting the optical axis 24. The lens 23 reversely Fourier-transforms the filtered light patterns of the illuminated dies 2 on the semiconductive wafer 1, and forms the images of the defects of the dies 2 in the rearward focal plane, i.e., the image plane, of the lens 23.
A photodetector array 26 is disposed about the optical axis 24 on the rearward focal plane of the lens 23, i.e., the image plane 25 on which the images of the defects are formed, and the respective light receiving elements of the photodetector array 26 receive the images of the defects present in the dies 2 on the optical axis 24.
In such prior art, to make electronic or optical noise sufficiently small, the design conditions of the Fourier transform lens 10 and reverse Fourier transform lens 23 become very severe.
To make optical noise sufficiently small, a severe limitation is imposed on the minimum spot diameter on the Fourier transform plane 17 and the minimum spot diameter on the image plane 25. Also, to make electronic noise sufficiently small, the lens 10 and lens 23 must be designed such that light diffracted in a telecentric cone of .+-.15.degree. or greater from any point in the field of view located on the object plane (forward focal plane) 14 of the lens is made into parallel rays of light with sufficiently small aberrations and a paraxial diffracted light image is finally formed with very small geometric strain.
From the conditions as described above, as shown in FIGS. 35A and 35B, in the illuminated area 15 on the semiconductive wafer 1, an observation field 27 which is a field of view in which the defect can be detected with sufficiently small electronic or optical noise becomes small as compared with the illuminated area 15. Also, the optimum illuminated area when exposing the space filter 21 is the illuminated area 15, and also when observing only the interior of the observation field 27, the illuminated area 15 of the same dimensions must be illuminated. The reason is that the Fourier transform patterns of the circuit patterns of the illuminated dies 2 need be made the same both during the making of the space filter 21 and during the defect inspecting operation.
With regard to this, as shown, for example, in FIG. 37 of the accompanying drawings, suppose an inspecting apparatus for illuminating the same area as the observation area 27 of FIGS. 35A and 35B. That is, in FIG. 37, a parallel beam of light 29 of a small diameter emerging from the Fourier transform lens 10 illuminates only the interior of the observation field. In this case, the useless illuminated area (the hatched portion of FIGS. 35A and 35B) created in the illuminated area 15 of FIGS. 35A and 35B is not created, but yet the diameter of a beam spot 31 formed in the Fourier transform plane 17 by the reflected light 30 from the wafer 1A of FIG. 37 having no circuit becomes large as compared with the diameter of the beam spot 20 in the case of FIG. 36, and this light cannot completely be intercepted by the space filter on the Fourier transform plane 17. Therefore, it is necessary to illuminate the illuminated area 15 during the detect inspection as well, and it has been difficult to apply illuminating light of sufficient illuminance to the interior of the observation field 27.
Also, the illuminance of the illuminating light in the observation field 27 shown in FIG. 35B affects the inspection time. That is, the observation field 27 is not so large as to enable the whole of each die 2 on the semiconductive wafer 1 to be observed at a time and therefore, it becomes necessary to move the observation field 27 relative to the dies 2 in the respective dies 2. The relative movement velocity depends on the light cumulation time of the photodetector array 26 and the luminance in the observation field 27, and as the luminance becomes lower, the relative movement velocity also becomes lower as a matter of course. Also, in some cases, a light cumulation method such as time delay integration (TDI) is used in the photodetector array 26, but in such cases, the requirement for the accuracy of the relative movement becomes severe.
In the prior art as described above, the light from the errorless pattern of each die 2 on the semiconductive wafer 1 is intercepted by the space filter 21, whereby only the light from the defective portion of each die 2 may be directed to the photodetector array 26. However, there has been the inconvenience that it is difficult to expose, for example, a photographic dry plate properly and thereby prepare a space filter 21 capable of effecting defect detection accurately. Also this space filter 21 must strictly be prepared for each circuit pattern on the semiconductive wafer 1, and this has been very cumbersome.